/Engine/Source/Runtime/Core/Public/Containers/UnrealString.h
Joins a range of 'something that can be concatentated to strings with +=' together into a single string with separators.
The final, joined, separated string.
template<typename RangeType> static [FString](API\Runtime\Core\Containers\FString) Join ( const RangeType & Range, const TCHAR * Separator )